Mate, people that own the cars have tried to give you advice. You have had people give their opinions, another guy saying he went to buy it but there were suspicious circumstances but you are still keen on it. You now need to make a decision. Asking the same questions on here is not going to get you anywhere.
The RAC/AA want near £400 to do an inspection which on a CAT damaged car would give you some piece of mind. Add that to the price you paying and tax difference etc and any value you are looking for is lost.
The cars are expensive to run and replacement parts aren't cheap either. If you can afford a little extra outlay initially, you may avoid problems later. With the CAT car you are taking a gamble and it is your decision to make. Nobody on here can reassure you and tell you everything is going to be alright.
Good luck with your decision!